Lithium ion secondary battery

ABSTRACT

A lithium ion secondary battery having more improved cycle characteristics is provided. The present invention relates to a lithium ion secondary battery having a negative electrode comprising a graphite and a silicon oxide having a composition represented by SiO x  (0&lt;x≤2), wherein AG/AS is within a range of 0.6 or more and 1.6 or less when a particle number average aspect ratio of the graphite is defined as AG and a particle number average aspect ratio of the silicon oxide is defined as AS.

TECHNICAL FIELD

The present invention relates to a lithium ion secondary battery, amethod for manufacturing the same, a vehicle and a power storage systemusing the lithium ion secondary battery.

BACKGROUND ART

Lithium ion secondary batteries are characterized by their small sizeand large capacity and are widely used as power sources for electronicdevices such as mobile phones and notebook computers, and havecontributed to the improvement of the convenience of portable ITdevices. In recent years, attention has also been drawn to the use inlarge-sized applications such as drive power supplies for motorcyclesand automobiles, and storage batteries for smart grids. As the demandfor lithium ion secondary batteries has increased and they are used invarious fields, batteries have been required to have characteristics;such as further higher energy density, lifetime characteristics that canwithstand long-term use, and usability under a wide range of temperatureconditions.

In general, carbon-based materials have been used for a negativeelectrode of the lithium-ion secondary battery, but in order to increasethe energy density of the battery, the use of silicon-based materialshaving a large capacity of absorbing and desorbing lithium ions per unitvolume has been studied for the negative electrode. However, thesilicon-based material deteriorates due to expansion and contractionthat are repeated by charging and discharging lithium, and thereforehave a problem in the cycle characteristics of the battery.

Various proposals have been made to improve the cycle characteristics ofthe lithium ion secondary battery using a silicon-based material for anegative electrode. Patent Document 1 discloses a method of improvingthe charge and discharge cycle life of a non-aqueous electrolytesecondary battery by mixing silicon oxide with elemental silicon andfurther covering its periphery with amorphous carbon to relax theexpansion and contraction of the electrode active material itself.Patent Document 2 discloses that, by specifying the size ratio ofsilicon oxide particles and graphite particles in a negative electrodecomprising silicon oxide and graphite, the silicon oxide particles aredisposed within spaces formed by graphite particles to suppress thechange in the volume of the entire negative electrode even when thesilicon oxide expands, and thus, the deterioration of the cyclecharacteristics can be suppressed.

CITATION LIST Patent Document

Patent Document 1: Japanese Patent Laid-Open Publication No. 2008-153117

Patent Document 2: Japanese Patent Laid-Open Publication No. 2013-101921

SUMMARY OF INVENTION Technical Problem

However, there is a problem that the discharge capacity decreases byrepeating charge and discharge cycles even in the lithium ion secondarybattery described in the above-mentioned prior art documents, and thus,further improvement of the cycle characteristics is required.

An object of the present invention is to provide a lithium ion secondarybattery that solves the above-mentioned problems.

Solution to Problem

The lithium ion secondary battery of the present invention is a batteryhaving a negative electrode comprising a graphite and a silicon oxidehaving a composition represented by SiO_(x) (0<x≤2), wherein AG/AS iswithin a range of 0.6 or more and 1.6 or less when a particle numberaverage aspect ratio of the graphite is defined as AG and a particlenumber average aspect ratio of the silicon oxide is defined as AS.

Advantageous Effect of Invention

According to the present invention, a lithium ion secondary batteryhaving more improved cycle characteristics is provided.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is an SEM-observed-picture (5,000 magnifications) of a negativeelectrode cross section in which the aspect ratio (AG) of the artificialgraphite is 2.0 and the aspect ratio (AS) of SiO is 3.0.

FIG. 2 is an SEM-observed-picture (5,000 magnifications) of a negativeelectrode cross section in which the aspect ratio (AG) of the artificialgraphite is 1.0 and the aspect ratio (AS) of SiO is 1.0.

FIG. 3 is an exploded perspective view showing a basic structure of afilm package battery.

FIG. 4 is a cross-sectional view schematically showing a cross sectionof the battery of FIG. 3.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described for eachconstituting member of the lithium ion secondary battery.

<Negative Electrode>

The negative electrode has a structure in which a negative electrodeactive material is laminated on a current collector as a negativeelectrode active material layer integrated by a negative electrodebinder. The negative electrode active material is a material capable ofreversibly absorbing and desorbing lithium ions with charge anddischarge.

The negative electrode active material comprises graphite and siliconoxide, which respectively exist as particles in the negative electrode.

The graphite used in this embodiment may be either natural graphite orartificial graphite. The shape of the graphite is not particularlylimited and may be any shape. Examples of the natural graphite includeflake-like graphite, scaly graphite, earthy graphite and the like, andexamples of the artificial graphite include massive artificial graphite,flake-like artificial graphite, and spherical artificial graphite suchas MCMB (mesophase microbeads). The graphite to be used may be coatedwith a carbon material or the like.

The graphite to be used preferably has a ratio (G/D) in the range of 2.0to 5.0, wherein the ratio (G/D) is a ratio of intensity G of a peak at1550 to 1650 cm⁻¹ to intensity D of a peak at 1300 to 1400 cm⁻¹ by Ramanspectroscopic analysis. Here, the peak intensity G is assigned tocrystalline carbon, the peak intensity D is assigned to amorphouscarbon, and the higher G/D ratio means that the graphite has highercrystallinity. By adjusting the G/D ratio of the graphite to be used inthe range of 2.0 to 5.0, it is possible to obtain graphite which canfollow the expansion and contraction of the silicon oxide at the time ofcharging and discharging, and therefore it is preferable in the presentembodiment. Graphite having G/D in the range of 2.0 to 5.0 means thatthe degree of graphitization in the negative electrode active materialis high.

The graphite particles are contained in an amount of preferably 50% bymass, more preferably 70% by mass or more and 97% by mass or less, inthe negative electrode active material.

The silicon oxide used in the present invention has a compositionrepresented by SiO_(x) (where 0<x≤2). A particularly preferred siliconoxide is SiO. The silicon oxide particles may be coated with a carbonmaterial or the like. In general, a carbon-coated silicon oxide particlecan provide a secondary battery having excellent cycle characteristics,but according to the present invention, cycle characteristics can befurther improved. The silicon oxide particles are contained in thenegative electrode active material in an amount of preferably 1% by massor more and 20% by mass or less, more preferably 3% by mass or more and10% by mass or less.

In the present invention, AG/AS is within a range of 0.6 or more and 1.6or less, preferably 0.8 or more and 1.2 or less, and particularlypreferably 0.9 or more and 1.1 or less in the negative electrode when anaspect ratio of the graphite particles on a number average of theparticles is defined as AG and an aspect ratio of the silicon oxideparticles on a number average of the particles is defined as AS. As thegraphite particles and the silicon oxide particles have close aspectratios, it is presumed that the graphite particles are less susceptibleto interference due to the expansion and contraction occurring duringcharging and discharging of the silicon oxide particles. Therefore, inthe present invention, damage of the graphite particles and the coatingthereof is suppressed to improve the cycle characteristics. The aspectratio of the graphite particles and the silicon oxide particles can beadjusted by particle shape control with shape separation and crushingtreatment of the particles.

The aspect ratio is a length ratio of the long axis direction to theshort axis direction in a particle and for example, it can be confirmedby observing a particle section with an electron microscope such as SEM(Scanning electron microscope). A particle number average value of theratio of the longest diameter to the shortest diameter of each particlein a particle section can be employed as the aspect ratio.

In the present invention, it is preferable for the cycle characteristicsof the battery that both of the aspect ratios of the graphite particlesand the silicon oxide particles are higher. Preferably, the particlenumber average aspect ratios of the graphite particles and the siliconoxide particles are respectively 2 or more, and more preferably, theparticle number average aspect ratios of the graphite particles and thesilicon oxide particles are respectively 3 or more.

In the present embodiment, the cycle characteristics may be furtherimproved by controlling the particle diameter in addition to theparticle shape. When D_(50G) is the median diameter of the graphiteparticles, and D_(50S) is the median diameter of the silicon oxideparticles. It is preferable that ranges of each median diameter satisfy:

5.0 μm<D_(50G)<25.0 μm,

1.0 μm<D_(50S)<15.0 μm,

0.5 μm<D_(50H)<15.0 μm.

In addition, it is also preferable that D_(50H)/D_(50S) is within arange of 0.6 to 5.0. By setting the particle sizes within the aboverange, preferable cycle characteristics can be obtained. This is becausethe impregnation and permeability property of the electrolytic solutionare particularly improved within the above range. Because theimpregnation property of the electrolytic solution is particularlyexcellent, it is presumed that the effect of an additive in theelectrolytic solution is readily realized. Measurement of the mediandiameter may be carried out by a laser diffraction/scattering typeparticle size distribution measuring device.

A negative electrode active material other than the graphite and thesilicon oxide may be additionally used in the negative electrode. Theadditional negative electrode active material is not limited, and knownmaterials may be used, and the examples thereof include silicon-basedmaterials such as silicon alloys, silicon composite oxides, and silicon,nitride; carbon-based materials such as amorphous carbon, and carbonnanotube; metals such as Al, Pb, Sn, In, Bi, Ag, Ba, Ga, Hg, Pd, Pt, Te,Zn, La and alloys thereof; and metal oxides such as aluminum oxide, tinoxide, indium oxide, zinc oxide, and lithium oxide. These can be usedalone or in combination of two or more.

Examples of the negative electrode binder include polyvinylidenefluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidenefluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene,

polypropylene, polyethylene, polyimide, polyamideimide and the like. Inaddition to the above, styrene butadiene rubber (SBR) and the like canbe used. When an aqueous binder such as an SBR emulsion is used, athickener such as carboxymethyl cellulose (CMC) can also be used. Theamount of the negative electrode binder is preferably 0.5 to 20 parts bymass based on 100 parts by mass of the negative electrode activematerial, from the viewpoint of the sufficient binding strength and thehigh energy density being in a trade-off relation with each other. Theabove-mentioned binders for a negative electrode may be mixed and used.

The negative electrode active material may be used together with aconductive assisting agent. Specific examples of the conductiveassisting agent are the same as those specifically exemplified in thepositive electrode, and the usage amount thereof may be the same.

As the negative electrode current collector, from the view point ofelectrochemical stability, aluminum, nickel, copper, silver, and alloysthereof are preferred. As the shape thereof, foil, flat plate, mesh andthe like are exemplified.

Examples of a method for forming the negative electrode active materiallayer include a doctor blade method, a die coater method, a CVD method,a sputtering method, and the like. It is also possible that, afterforming the negative electrode active material layer in advance, a thinfilm of aluminum, nickel or an alloy thereof may be formed by a methodsuch as vapor deposition, sputtering or the like to obtain a negativeelectrode current collector.

<Positive Electrode>

The positive electrode includes a positive electrode active materialcapable of reversibly absorbing and desorbing lithium ions with chargeand discharge and it has a structure in which the positive electrodeactive material is laminated on a current collector as a positiveelectrode active material layer integrated by a positive electrodebinder.

The positive electrode active material in the present embodiment is sotparticularly limited as long as it is a material capable of absorb anddesorb lithium, but from the viewpoint of high energy density, acompound having high capacity is preferably contained. Examples of thehigh capacity compound include lithium nickel composite oxides in whicha part of the Ni of lithium nickelate (LiNiO₂) is replaced by anothermetal element, and layered lithium nickel composite oxides representedby the following formula (A) are preferred.

Li_(y)Ni_((1−x))M₂O₂   (A)

wherein 0≤x<1, 0<y≤1.20, and M is at least one element selected from thegroup consisting of Co, Al, Mn, Fe, Ti, and B.

In addition, from the viewpoint of high capacity, it is preferred thatthe content of Ni is high, that is, x is less than 0.5, furtherpreferably 0.4 or less in the formula (A). Examples of such compoundsinclude Li_(α)Ni_(β)Co_(γ)Mn_(δ)O₂ (1≤α≤1.2, β+γ+δ=1, β≥0.7, and γ≤0.2)and Li_(α)Ni_(β)Co_(γ)Al_(δ)O₂ (1≤α≤1.2, β+γ+δ=1, β≥0.7, and γ≤0.2) andparticularly include LiNi_(β)Co_(γ)Mn_(δ)O₂ (0.75≤β≤0.85, 0.05≤γ≤0.15,and 0.10≤δ≤0.20). More specifically, for example,LiNi_(0.8)Co_(0.05)Mn_(0.15)O₂, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂,LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, and LiNi_(0.8)Co_(0.1)Al_(0.1)O₂ may bepreferably used.

From the viewpoint of thermal stability, it is also preferred that thecontent of Ni does not exceed 0.5, that is, x is 0.5 or more in theformula (A). In addition, it is also preferred that particulartransition metals do not exceed half. Examples of such compounds includeLi_(α)Ni_(β)Co_(γ)Mn_(δ)O₂ (1≤α≤1.2, β+γ+δ=1, 0.2≤β≤0.5, 0.1≤γ≤0.4, and0.1≤δ≤0.4). More specific examples may includeLiNi_(0.4)Co_(0.8)Mn_(0.3)O₂ (abbreviated as NCM433),LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ (abbreviatedas NCM523), and LiNi_(0.5)Co_(0.8)Mn_(0.2)O₂ (abbreviated as NCM532)(also including those in which the content of each transition metalfluctuates by about 10% in these compounds).

In addition, two or more compounds represented by the formula (A) may bemixed and used, and, for example, it is also preferred that NCM532 orNCM523 and NCM433 are mixed in the range of 9:1 to 1:9 (as a typicalexample, 2:1) and used. Further, by mixing a material in which thecontent of Ni is high (x is 0.4 or less in the formula (A)) and amaterial in which the content of Ni does not exceed 0.5 (x is 0.5 ormore, for example, NCM433), a battery having high capacity and highthermal stability can also be formed.

Examples of the positive electrode active materials other than the aboveinclude lithium manganate having a layered structure or a spinelstructure such as LiMnO₂, Li_(x)Mn₂O₄ (0<x<2), Li₂MnO₃, andLi_(x)Mn_(1.5)Ni_(0.5)O₄ (0<x<2); LiCoO₂ or materials in which a part ofthe transition metal in this material is replaced by other metal(s);materials in which Li is excessive as compared with the stoichiometriccomposition in these lithium transition metal oxides; materials havingolivine structure such as LiMPO₄, and the like. In addition, materialsin which a part of elements in these metal oxides is substituted, by Al,Fe, P, Ti, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, La arealso usable. The positive electrode active materials described above maybe used alone or in combination of two or more.

As the positive electrode binder, the same binder as the negativeelectrode binder can be used. Among them, polyvinylidene fluoride orpolytetrafluoroethylene is preferable from the viewpoint of versatilityand low cost, and polyvinylidene fluoride is more preferable. The amountof the positive electrode binder is preferably 2 to 10 parts by massbased on 100 parts by mass of the positive electrode active material,from the viewpoint of the binding strength and energy density that arein a trade-off relation with each other.

For the coating layer containing the positive electrode active material,a conductive assisting agent may be added for the purpose of loweringthe impedance. Examples of the conductive assisting agent includeflake-like, soot, and fibrous carbon fine particles and the like, forexample, graphite, carbon black, acetylene black, vapor grown carbonfibers (for example, VGCF manufactured by Showa Denko) and the like.

As the positive electrode current collector, the same material as thenegative electrode current collector can be used. In particular, as thepositive electrode, a current collector using aluminum, an aluminumalloy, or iron-nickel-chromium-molybdenum based stainless steel ispreferable.

Similar to the negative electrode, the positive electrode may beprepared by forming a positive electrode active material layercontaining a positive electrode active material and a binder forpositive electrode on a positive electrode current collector.

<Electrolyte Solution>

The electrolyte solution of the lithium ion secondary battery accordingto the present embodiment is not particularly limited, but is preferablya nonaqueous electrolyte solution containing a nonaqueous solvent and asupporting salt that is stable at the operating potential of thebattery.

Examples of nonaqueous solvents include aprotic organic solvents, forexamples, cyclic carbonates such as propylene carbonate (PC), ethylenecarbonate (EC) and butylene carbonate (BC); open-chain carbonates suchas dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methylcarbonate (EMC) and dipropyl carbonate (DPC); aliphatic carboxylic acidesters such as propylene carbonate derivatives, methyl formate, methylacetate and ethyl propionate; ethers such as diethyl ether and ethylpropyl ether; phosphoric acid esters such as trimethyl phosphate,triethyl phosphate, tripropyl phosphate, trioctyl phosphate andtriphenyl phosphate; and fluorinated aprotic organic solvents obtainableby substituting at least a part of the hydrogen atoms of these compoundswith fluorine atom(s), and the like.

Among them, cyclic or open-chain carbonate(s) such as ethylene carbonate(EC), propylene carbonate (PC), butylene carbonate (BC), dimethylcarbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC),dipropyl carbonate (DPC) and the like is preferably contained.

Nonaqueous solvent may be used alone, or in combination of two or more.

The supporting salts include lithium salts, such as LiPF₆, LiAsF₆,LiAlCl₄, LiClO₄, LiBF₄, LiSbF₆, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₈, andLiN(CF₃SO₂)₂. Supporting salts may be used alone or in combination oftwo or more. From the viewpoint of cost reduction, LiPF₆ is preferable.

The electrolyte solution may further contain additives. The additive isnot particularly limited, and examples thereof include halogenatedcyclic carbonates, unsaturated cyclic carbonates, cyclic or open-chaindisulfonic acid esters, and the like. The addition of these compoundsimproves battery characteristics such as cycle characteristics. This ispresumably because these additives decompose during charging anddischarging of the lithium ion secondary battery to form a film on thesurface of the electrode active material and inhibit decomposition ofthe electrolyte solution and supporting salt. It is considered that thefilm formation effect on the surface of the negative electrode by theadditive is further enhanced due to the effect of preventing the damageof the coating on the graphite surface in the present invention.Therefore, the cycle characteristics may be further improved byadditives in some cases. The additives listed above are specificallydescribed below.

As the halogenated cyclic carbonate, the examples thereof include acompound represented by the following formula (B).

In the formula (B), A, B, C and D each independently represent ahydrogen atom, a halogen atom, an alkyl group or a halogenated alkylgroup having 1 to 6 carbon atoms, and at least one of A, B, C and D is ahalogen atom or a halogenated alkyl group. The alkyl group and thehalogenated alkyl group have preferably 1 to 4 carbon atoms, and morepreferably 1 to 3 carbon atoms.

In one embodiment, the halogenated cyclic carbonate is preferably afluorinated cyclic carbonate. The examples of the fluorinated cycliccarbonates include compounds in which a part or all of the hydrogenatoms of ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate (BC) and the like are substituted with fluorine atoms, and thelike. Among these, 4-fluoro-1,3-dioxolan-2-one (fluoroethylenecarbonate: FEC) is preferred.

The content of the fluorinated cyclic carbonate is not particularlylimited, but it is preferably 0.01% by mass or more and 1% by mass orless in the electrolytic solution. When it is contained in an amount of0.01% by mass or more, a sufficient film forming effect can be obtained.When the content is 1% by mass or less, gas generation due todecomposition of the fluorinated cyclic carbonate itself can be reduced.In the present embodiment, the content is more preferably 0.8% by massor less. By setting the content of the fluorinated cyclic carbonate to0.8% by mass or less, it is possible to suppress the decrease in theactivity of the negative electrode active material and maintain goodcycle characteristics.

Unsaturated cyclic carbonates are cyclic carbonates having at least onecarbon-carbon unsaturated bond in a molecule, and the examples thereofinclude vinylene carbonate compounds such as vinylene carbonate, methylvinylene carbonate, ethyl vinylene carbonate, 4,5-dimethyl vinylenecarbonate, 4,5-diethyl vinylene carbonate; vinyl ethylene carbonatecompounds such as 4-vinyl ethylene carbonate, 4-methyl-4-vinyl ethylenecarbonate, 4-ethyl-4-vinyl ethylene carbonate, 4-n-propyl-4-vinyleneethylene carbonate, 5-methyl-4-vinyl ethylene carbonate, 4,4-divinylethylene carbonate, 4,5-divinyl ethylene carbonate,4,4-dimethyl-5-methylene ethylene carbonate, 4,4-diethyl-5-methyleneethylene carbonate; and the like. Among these, vinylene carbonate and4-vinylethylene carbonate are preferable, and vinylene carbonate isparticularly preferable.

The content of the unsaturated cyclic carbonate is not particularlylimited, but it is preferably 0.01% by mass or more and 10% by mass orless in the electrolytic solution. When it is contained in an amount of0.01% by mass or more, a sufficient film forming effect can be obtained.When the content is 10% by mass or less, gas generation due to thedecomposition of the unsaturated cyclic carbonate itself can be reduced.In the present embodiment, from the viewpoint of suppressing thedecrease in the activity of the negative electrode active material, itis more preferably 5% by mass or less.

As the cyclic or open-chain disulfonic acid esters, cyclic disulfonicacid esters represented by the following formula (C) or open-chaindisulfonic acid esters represented by the following formula (D) can beexemplified.

In the formula (C), R₁ and R₂, independently each other, represent asubstituent selected from the group consisting of a hydrogen atom, analkyl group having 1 to 5 carbon atoms, a halogen group, and an aminogroup, R₃ represents an alkylene group having 1 to 5 carbon atoms, acarbonyl group, a sulfonyl group, a fluoroalkylene group having 1 to 6carbon atoms, or a divalent group having 2 to 6 carbon atoms in whichalkylene units or fluoroalkylene units are bonded via ether group.

In the formula (C), R₁ and R₂ are each independently preferably ahydrogen atom, an alkyl group having 1 to 3 carbon atoms or a halogengroup, and R₃ is more preferably an alkylene group or fluoroalkylenegroup having 1 or 2 carbon atoms.

Preferable examples of the cyclic disulfonic acid esters represented bythe formula (C) include compounds represented by the following formulae(1) to (20).

In the formula (D), R⁴ and R⁷, independently each other, represent anatom or a group selected from the group consisting of a hydrogen atom,an alkyl group having 1 to 5 carbon atoms, an alkoxy group having 1 to 5carbon atoms, an fluoroalkyl group having 1 to 5 carbon atoms, anpolyfluoroalkyl group having 1 to 5 carbon atoms, —SO₂X₃ (X₃ is an alkylgroup having 1 to 5 carbon atoms), —SY₁ (Y₁ is an alkyl group having 1to 5 carbon atoms), —COZ (Z is a hydrogen atom or an alkyl group having1 to 5 carbon atoms), and a halogen atom. R⁵ and R⁶, independently eachother, represent an atom or a group selected from an alkyl group having1 to 5 carbon atoms, an alkoxy group having 1 to 5 carbon atoms, aphenoxy group, a fluoroalkyl group having 1 to 5 carbon atoms, apolyfluoroalkyl group having 1 to 5 carbon atoms, a fluoroalkoxy grouphaving 1 to 5 carbon atoms, a polyfluoroalkoxy group having 1 to 5carbon atoms, a hydroxyl group, a halogen atom, —NX₄X₅ (X₄ and X₅ are,independently each other, a hydrogen atom or an alkyl group having 1 to5 carbon atoms) or —NY₂CONY₃Y₄ (Y₂ to Y₄ are, independently each other,a hydrogen atom or an alkyl group having 1 to 5 carbon atoms).

In the formula (D), R⁴ and R⁷ are, independently each other, preferablya hydrogen atom, an alkyl group having 1 or 2 carbon atoms, afluoroalkyl group having 1 or 2 carbon atoms, or a halogen atom, and R⁵and R⁶, independently each other, more preferably represent an alkylgroup having 1 to 3 carbon atoms, an alkoxy group having 1 to 3 carbonatoms, a fluoroalkyl group having 1 to 3 carbon atoms, a polyfluoroalkylgroup having 1 to 3 carbon atoms, a hydroxyl group or a halogen atom.

Preferred compounds of the open-chain disulfonic acid ester compoundrepresented by the formula (D) include, for example, the followingcompounds.

The content of the cyclic or open-chain disulfonic acid ester ispreferably 0.005 mol/L or more and 10 mol/L or less in the electrolytesolution, more preferably 0.01 mol/L or more and 5 mol/L or less, andparticularly preferably 0.05 mol/L or more and 0.15 mol/L or less. Whenit is contained in an amount of 0.005 mol/L or more, a sufficient filmforming effect can be obtained. When the content is 10 mol/L or less, itis possible to suppress an increase in the viscosity of the electrolytesolution and an increase thereby in resistance.

Additives may be used alone or in combination of two or more. When twoor more kinds of additives are used in combination, the total content ofthe additives is preferably 10% by mass or less, more preferably 5% bymass or less in the electrolyte solution.

<Separator>

The separator may be of any type as long as it suppresses electronconduction between the positive electrode and the negative electrode,does not inhibit the permeation of charged substances, and hasdurability against the electrolyte solution. Specific examples of thematerial include polyolefins such as polypropylene and polyethylene;cellulose, polyethylene terephthalate, polyimide, polyvinylidenefluoride and aromatic polyamides (aramid) such as polymetaphenyleneisophthalamide, polyparaphenylene terephthalamide andcopolyparaphenylene 3,4′-oxydiphenylene terephthalamide; and the like.These can be used as porous films, woven fabrics, nonwoven fabrics andthe like.

<Method for Producing Lithium Ion Secondary Battery>

The lithium ion secondary battery according to the present embodimentcan be manufactured according to conventional method. An example of amethod for manufacturing a lithium ion secondary battery will bedescribed taking a stacked laminate type lithium ion secondary batteryas an example. First, in the dry air or an inert atmosphere, thepositive electrode and the negative electrode are placed to oppose toeach other via a separator to form the above-mentioned electrodeelement. Next, this electrode element is accommodated in an outerpackage (container), an electrolyte solution is injected, and theelectrode is impregnated with the electrolyte solution. Thereafter, theopening of the outer package is sealed to complete the lithium ionsecondary battery.

The lithium ion secondary battery according to the present embodimentmay be, for example, a secondary battery having a structure as shown inFIGS. 2 and 3. This secondary battery comprises a battery element 20, afilm package 10 housing the battery element 20 together with anelectrolyte, and a positive electrode tab 51 and a negative electrodetab 52 (hereinafter these are also simply referred to as “electrodetabs”).

In the battery element 20, a plurality of positive electrodes 30 and aplurality of negative electrodes 40 are alternately stacked withseparators 25 sandwiched therebetween as shown in FIG. 4. In thepositive electrode 30, an electrode material 32 is applied to bothsurfaces of a metal foil 31, and also in the negative electrode 40, anelectrode material 42 is applied to both surfaces of a metal foil 41 inthe same manner.

As shown in FIG. 3, the secondary battery may have an arrangement inwhich the electrode tabs are drawn out to one side of the outer package,but the electrode tab may be drawn out to both sides of the outerpackage. Although detailed illustration is omitted, the metal foils ofthe positive electrodes and the negative electrodes each have anextended portion in a part of the outer periphery. The extended portionsof the negative electrode metal foils are brought together into one andconnected to the negative electrode tab 52, and the extended portions ofthe positive electrode metal foils are brought together into one andconnected to the positive electrode tab 51 (see FIG. 4). The portion inwhich the extended portions are brought together into one in thestacking direction in this manner is also referred to as a “currentcollecting portion” or the like.

The film package 10 is composed of two films 10-1 and 10-2 in thisexample. The films 10-1 and 10-2 are heat-sealed to each other in theperipheral portion of the battery element 20 and hermetically sealed. InFIG. 4, the positive electrode tab 51 and the negative electrode tab 52are drawn out in the same direction from one short side of the filmpackage 10 hermetically sealed in this manner.

Of course, the electrode tabs may be drawn out from different two sidesrespectively. In addition, regarding the arrangement of the films, inFIG. 3 and FIG. 4, an example in which a cup portion is formed in onefilm 10-1 and a cup portion is not formed in the other film 10-2 isshown, but other than this, an arrangement in which cup portions areformed in both films (not illustrated), an arrangement in which a cupportion is not formed in either film (not illustrated), and the like mayalso be adopted.

<Assembled Battery>

A plurality of lithium ion secondary batteries according to the presentembodiment may be combined to form an assembled battery. The assembledbattery may be configured by connecting two or more lithium ionsecondary batteries according to the present embodiment in series or inparallel or in combination of both. The connection in series and/orparallel makes it possible to adjust the capacitance and voltage freely.The number of lithium ion secondary batteries included in the assembledbattery can be set appropriately according to the battery capacity andoutput.

<Vehicle>

The lithium ion, secondary battery or the assembled battery according tothe present embodiment can be used in vehicles. Vehicles according to anembodiment of the present invention include hybrid vehicles, fuel cellvehicles, electric vehicles (besides four-wheel vehicles (cars, trucks,commercial vehicles such as buses, light automobiles, etc.) two-wheeledvehicle (bike) and tricycle), and the like. The vehicles according tothe present embodiment is not limited to automobiles, it may be avariety of power source of other vehicles, such as a moving body like atrain.

<Power Storage Equipment>

The lithium ion secondary battery or the assembled battery according tothe present embodiment can be used in power storage system. The powerstorage systems according to the present embodiment include, forexample, those which is connected between the commercial power supplyand loads of household appliances and used, as a backup power source oran auxiliary power in the event of power outage or the like, or thoseused as a large scale power storage that stabilize power output withlarge time variation supplied by renewable energy, for example, solarpower generation.

EXAMPLE Example 1

<Manufacturing of Lithium Ion Secondary Battery>

Polyvinylidene fluoride (PVdF) as a binder in an amount of 3% by massbased on the mass of the positive electrode active material, and alayered lithium nickel composite oxide (LiNi_(0.8)Co_(0.15)Al_(0.05)O₂)in a remaining amount other than the above, are dispersed uniformly inNMP using a rotation revolution type three-axis mixer excellent instirring and mixing, to prepare a positive electrode slurry. Thepositive electrode slurry was uniformly applied to a positive electrodecurrent collector of aluminum foil with a thickness of 20 μm using acoater. After drying by evaporating NMP, the back side was also coatedin the same way. After drying, the density was adjusted by roll press,to prepare positive electrode active material layers on both sides ofthe current collector. The mass per unit area of the positive electrodeactive material layer was 50 mg/cm².

Artificial graphite (G/D ratio=4.8) and SiO (G/D ratio=0.84) weredispersed uniformly in an aqueous solution containing 1% by mass of CMC(carboxymethyl cellulose) using a rotation revolution type three-axismixer excellent in stirring and mixing so that the mixing ratio thereofin the negative electrode active material layer was 95:5. Then, usingSBR binder (the content in the negative electrode was 2% by mass) as abinder, negative electrode slurry was prepared. The negative electrodeslurry was uniformly applied to a negative electrode current collectorof copper foil with a thickness of 10 μm using a coater. After drying byevaporating water, the back side was also coated in the same way. Afterdrying, the density was adjusted by roll press, to prepare negativeelectrode active material layers on both sides of the current collector.The mass per unit area of the negative electrode active material layerwas 20 mg/cm².

For the electrolyte solution, 1 mol/L of LiPF₆ as an electrolyte wasdissolved in a solvent of ethylene carbonate (EC): diethyl carbonate(DEC)=30.70 (vol %).

The resulting positive electrode was cut into 13 cm×7 cm, and thenegative electrode was cut into 12 cm×6 cm. The both surfaces of thepositive electrode was covered by a polypropylene separator of 14 cm×8cm, the negative active material layer was disposed thereon so as toface the positive electrode active material layer, to prepare anelectrode stack. Next, the electrode stack was sandwiched by two sheetsof aluminum laminate film of 15 cm×9 cm, the three sides excluding oneside of long side were heat sealed with a seal width of 8 mm. Afterinjecting the electrolyte solution, the remaining side was heat sealed,to produce a laminate cell type battery.

<Measurement of Capacity Retention Ratio>

300 times of charge-discharge cycle test were performed in athermostatic oven at 45° C. to measure the capacity retention ratio andto evaluate the lifetime, in the charge, the secondary battery wascharged, at 1 C up to maximum voltage of 4.2 V and then subjected toconstant voltage charge at 4.2 V, and the total charge time was 2.5hours. In the discharge, the secondary battery was subjected to constantcurrent discharge at 1 C to 2.5 V. The capacity after thecharge-discharge cycle test was measured, and the ratio to the capacitybefore the charge-discharge cycle test was calculated. The results areshown in Table 1.

<Cross-Section Observation by SEM>

The negative electrode for measuring aspect ratio, which was prepared inthe same manner as used to manufacture the battery, was cut and itscross section was observed with SEM. Number average aspect ratios ofrandomly selected 20 SiO particles and randomly selected 20 artificialgraphite particles were respectively determined. The results are shownin Table 1.

<Measurement of Particle Size>

The median diameter was measured with a particle size distributionmeasuring apparatus by a laser diffraction scattering method. The 50%cumulative diameter D_(50G) of the artificial graphite material used inthe cell evaluation in this example was 14 μm, and the 50% cumulativediameter D_(50s) of the SiO material used in the cell evaluation in thisexample was 5 μm. The results are shown in Table 1.

<Raman Spectroscopic Analysis>

The crystallinity of the graphite material in the negative electrode wasmeasured using a laser Raman spectrometer. The excitation wavelength ofthe laser was set to 532.15 nm and the exposure time was set asaccumulation of twice of 20 seconds. The results of the G/D ratio areshown in Table 1.

Example 2 to 17

In each example, a negative electrode was produced in the same manner asin Example 1, using SiO particles and artificial graphite particles,which have different particle shape from Example 1 and have the aspectratios as described in Table 1. Also regarding the layered lithiumnickel composite oxide, lithium nickel composite oxides denoted, byLi_(α)Ni_(β)Co_(γ)Mn_(δ)O₂ or Li_(α)Ni_(β)Co_(γ)Al_(δ)O₂ was used as apositive electrode active material. In the lithium nickel compositeoxides, the molar ratio (%) of each metal element to 1 mol of Li was setas described in Table 1. Except for that, the batteries were prepared inthe same manner as in Example 1, and determination of capacity retentionratio and calculation of aspect ratios of the artificial graphite andSiO from cross section observation by SEM were respectively carried outin the same manner. In addition, the particle sizes of the artificialgraphite and SiO and Raman spectroscopic G/D ratios of the artificialgraphite were also measured in the same manner. The results are shown inTable 1 in addition to Example 1.

Comparative Examples 1 to 18

A negative electrode was produced in the same manner as in Example 1,using SiO particles and artificial graphite particles which have adifferent particle shape from Example 1 and have the aspect ratios asdescribed in Table 2. Except for that, the batteries were prepared inthe same manner as in Example 1, and determination of the capacityretention ratio and calculation of the aspect ratios of the artificialgraphite and SiO from cross section observation by SEM were respectivelycarried out in the same manner. In addition, the particle sizes of theartificial graphite and SiO and the Raman spectroscopic G/D ratios ofthe artificial graphite were also measured in the same manner. Theresults are shown in Table 2.

In the example in which the ratio of the aspect ratio of artificialgraphite particles (AG) to the aspect ratio of SiO particles (AS) iswithin the range of 0.6 to 1.6, the capacity maintenance ratio after 300cycles at 45° C. was higher than that of the comparative example, andtherefore the cycle characteristics tended to be improved. Among them,in Examples in which AG/AS was 1, the cycle characteristics were thebest.

TABLE 1 Mixing ratio of Aspect ratio negative Aspect Capacity Positiveelectrode ratio of Aspect retention Median electrode active materialsartificial ratio of ratio after diameter G/D ratio mixing ratioArtificial graphite SiO 300 cycles (μm) of graphite Samples Ni Co Mn Algraphite SiO (AG) (AS) AG/AS at 45° C. D_(S0G) D_(G08) material Example1 80 15 5 95 5 2.0 3.0 0.67 88 14 5 4.8 Example 2 80 15 5 95 5 1.0 1.50.67 88 14 5 4.8 Example 3 80 15 5 95 5 1.0 1.5 0.67 88 14 5 4.8 Example4 80 15 5 95 5 3.0 4.0 0.75 89 14 5 4.8 Example 5 80 15 5 95 5 3.0 2.50.80 89 14 5 4.8 Example 6 80 15 5 95 5 1.0 1.0 1.00 90 14 5 4.8 Example7 80 15 5 95 5 2.0 2.0 1.00 91 14 5 4.8 Example 8 80 15 5 95 5 3.0 3.01.00 92 14 5 4.8 Example 9 80 15 5 95 5 4.0 4.0 1.00 93 14 5 4.8 Example10 80 15 5 95 5 5.0 5.0 1.00 95 14 5 4.8 Example 11 80 10 10 95 5 3.03.0 1.00 94 14 5 4.8 Example 12 80 5 15 95 5 3.0 3.0 1.00 98 14 5 4.8Example 13 50 20 80 95 5 3.0 3.0 1.00 96 14 5 4.8 Example 14 84 33 33 955 3.0 3.0 1.00 97 14 5 4.8 Example 15 80 15 5 95 5 2.0 2.0 1.00 93 14 54.8 Example 16 80 15 5 95 5 1.6 1.0 1.60 88 14 5 4.8 Example 17 80 15 595 5 3.0 2.0 1.60 88 14 5 4.8

TABLE 2 Mixing ratio of Aspect ratio negative Aspect Capacity Positiveelectrode ratio of Aspect retention Median electrode active materialsartificial ratio of ratio after diameter G/D ratio mixing ratioArtificial graphite SiO 300 cycles (μm) of graphite Samples Ni Co Mn Algraphite SiO (AG) (AS) AG/AS at 45° C. D_(S0G) D_(G08) materialComparative 80 15 5 95 5 1.0 5.0 0.20 88 14 5 4.8 example 1 Comparative80 15 5 95 5 1.0 4.0 0.25 80 14 5 4.8 example 2 Comparative 80 15 5 95 51.0 4.0 0.25 80 14 5 4.8 example 3 Comparative 80 15 5 95 5 1.0 3.0 0.3382 14 5 4.8 example 4 Comparative 80 15 5 95 5 1.0 3.0 0.33 82 14 5 4.8example 5 Comparative 80 15 5 95 5 1.0 2.5 0.40 84 14 5 4.8 example 6Comparative 80 15 5 95 5 1.0 2.5 0.40 84 14 5 4.8 example 7 Comparative80 15 5 95 5 1.0 2.0 0.50 86 14 5 4.8 example 8 Comparative 80 15 5 95 52.0 4.0 0.50 86 14 5 4.8 example 9 Comparative 80 15 5 95 5 1.0 2.0 0.5086 14 5 4.8 example 10 Comparative 80 15 5 95 5 2.0 1.0 2.00 86 14 5 4.8example 11 Comparative 80 15 5 95 5 2.5 1.0 2.50 84 14 5 4.8 example 12Comparative 80 15 5 95 5 2.5 1.0 2.50 84 14 5 4.8 example 13 Comparative80 15 5 95 5 3.0 1.0 3.00 82 14 5 4.8 example 14 Comparative 80 15 5 955 3.0 1.0 3.00 82 14 5 4.8 example 15 Comparative 80 15 5 95 5 4.0 1.04.00 80 14 5 4.8 example 16 Comparative 80 15 5 95 5 4.0 1.0 4.00 80 145 4.8 example 17 Comparative 80 15 5 95 5 5.0 1.0 5.00 90 14 5 4.8example 18

INDUSTRIAL APPLICABILITY

The battery according to the present invention can be utilized in, forexample, all the industrial fields requiring a power supply and theindustrial fields pertaining to the transportation, storage and supplyof electric energy. Specifically, it can be used in, for example, powersupplies for mobile equipment such as cellular phones and notebookpersonal computers; power supplies for electrically driven vehiclesincluding an electric vehicle, a hybrid vehicle, an electric motorbikeand an electric-assisted bike, and moving/transporting media such astrains, satellites and submarines; backup power supplies for UPSs; andelectricity storage facilities for storing electric power generated byphotovoltaic power generation, wind power generation and the like.

EXPLANATION OF REFERENCE

-   10 film package-   20 battery element-   25 separator-   30 positive electrode-   40 negative electrode

1. A lithium ion secondary battery having a negative electrodecomprising a graphite and a silicon oxide having a compositionrepresented by SiO_(x) (0<x≤2), wherein AG/AS is within a range of 0.6or more and 1.8 or less when a particle number average aspect ratio ofthe graphite is defined as AG and a particle number average aspect ratioof the silicon oxide is defined as AS.
 2. The lithium ion secondarybattery according to claim 1, wherein D_(50G)/D_(50S) is within a rangeof 0.6 to 5.0 when a median diameter of the graphite is defined asD_(50G) and a median diameter of the silicon oxide is defined asD_(50S).
 3. The lithium ion secondary battery according to claim 1,wherein the graphite has a ratio (G/D) in a range of 2.0 to 5.0, whereinthe ratio (G/D) is a ratio of intensity G of a peak at 1550 to 1850 cm⁻¹to intensity D of a peak at 1300 to 1400 cm⁻¹ by Raman spectroscopicanalysis.
 4. A vehicle comprising the lithium ion secondary batteryaccording to claim 1, mounted thereon.
 5. (canceled)
 6. A method ofproducing a secondary battery comprising: a step of stacking a positiveelectrode and a negative electrode via a separator to produce anelectrode element and a step of enclosing the electrode element and anelectrolyte solution in an outer package, wherein the negative electrodecomprises a graphite and a silicon oxide having a compositionrepresented by SiO_(x) (0<x≤2), and AG/AS is within a range of 0.6 ormore and 1.6 or less when a particle number average aspect ratio of thegraphite is defined as AG and a particle number average aspect ratio ofthe silicon oxide is defined as AS.